A sheet material is required to have high flatness for assuring quality and for consistent production. In fulfilling this requirement, how the flatness is to be controlled properly in the production process of sheet material has conventionally been a challenge.
Generally, as an index indicative of flatness, difference in elongation percentage or degree of steepness is used.
The difference in elongation percentage Δε is a difference between an elongation percentage εCENT in a central portion in the width direction of a sheet material and an elongation percentage εEDGE in a portion other than the central portion in the width direction of the sheet material (generally, in the vicinity of the edge) as measured in a certain section in the longitudinal direction of the sheet material, and is represented by Formula (2).Δε=εCENT−εEDGE  (2)
Also, the degree of steepness λ is defined as λ=δ/P by using the height δ of standing wave of sheet and the pitch P thereof. In the case where the shape of the standing wave of sheet is approximated as a sinusoidal wave, the well-known relationship represented by Formula (3) exists between the difference in elongation percentage Δε and the degree of steepness λ (%).
                    λ        =                  {                                                                                          +                                          2                      π                                                        ⁢                                                                                  Δɛ                                                                                    1                      /                      2                                                        ×                  100                  ⁢                                                                          ⁢                                      (                                                                  when                        ⁢                                                                                                  ⁢                        Δɛ                                            ≧                      0                                        )                                                                                                                                            -                                          2                      π                                                        ⁢                                                                                  Δɛ                                                                                    1                      /                      2                                                        ×                  100                  ⁢                                                                          ⁢                                      (                                                                  when                        ⁢                                                                                                  ⁢                        Δɛ                                            <                      0                                        )                                                                                                          (        3        )            
For example, the production line for a hot-rolled steel sheet, which is one example of the sheet material, generally comprises a heating furnace, roughing mill, finishing mill train, cooling zone, and coiling machine. A starting material heated in the heating furnace is rolled by the roughing mill to produce a slab (rough bar) having a thickness of 30 to 60 mm. Next, the slab is rolled by the finishing mill train consisting of six to seven finishing mills to produce a hot-rolled steel sheet having a thickness required by the customer. This hot-rolled steel sheet is cooled by the cooling zone, and is coiled by the coiling machine.
The production of a hot-rolled steel sheet having high flatness is important for ensuring the product quality, for stably passing the steel sheet through the finishing mill and coiling it by using the coiling machine, and also for maintaining high productivity. Poor flatness of a hot-rolled steel sheet is caused by unevenness in the sheet width direction of elongation percentage produced in the finishing mill train and cooling zone. Therefore, as a method for producing a hot-rolled steel sheet having high flatness, there has been proposed a method in which a flatness meter or a sheet thickness profile meter is installed between the finishing mills or on the exit of the finishing mill train, and based on the measured value of the meter, the work roll bender of the finishing mill is feedback controlled, or a method in which the setup condition of the shift position of work roll or the load distribution of finishing mill train is controlled by learning. The above-described controlling method is described, for example, in JP11-104721A. Also, there has been proposed a method in which a flatness meter is installed on the exit of the cooling zone, and based on the measured value thereof, the amount of cooling water of cooling nozzles of the cooling zone is feedback controlled. To carry out the above-described controlling method, a method and a device for measuring the flatness of a hot-rolled steel sheet running at a high speed at a location between the finishing mills, on the exit of the finishing mill train, or on the exit of the cooling zone have been devised and used for actual machines.
As a conventional method for measuring the flatness of a hot-rolled steel sheet, there has been known a method in which a linear pattern consisting of a plurality of light lines extending in the sheet width direction is projected onto the surface of the hot-rolled steel sheet that has been hot rolled and is running, the linear pattern is photographed from the direction different from the linear pattern projecting direction by a two-dimensional camera, and based on distortion of the linear pattern in the picked-up image, the surface shape, and therefore the flatness of the hot-rolled steel sheet is measured. In this method, by projecting the linear pattern over the range of about 1 m in the longitudinal direction (rolling direction) of the hot-rolled steel sheet, the measurement accuracy in the state in which standing waves of sheet observed frequently at a location just close to the exit of the finishing mill exist steadily (at the fixed end because the standing waves of sheet are fixed by the finishing mill) is restrained from being deteriorated. The above-described flatness measuring method is described, for example, in JP61-40503A and JP2008-58036A.
JP61-40503A describes a method in which by scanning three laser beams, which are fired spacedly in the longitudinal direction of sheet, at a high speed in the sheet width direction, a linear pattern consisting of three light lines is projected onto the sheet surface, this linear pattern is photographed by a camera, and based on the distortion of the linear pattern in the picked-up image thus obtained, the surface shape, and therefore the flatness of the sheet is measured. However, the linear pattern consisting of three light lines does not allow highly accurate measurement of the sheet surface shape, so that there arises a problem that the measurement accuracy is extremely deteriorated especially when the period of standing waves of sheet is short.
JP2008-58036A describes a method in which a high-density linear pattern consisting of a plurality of light lines extending in the sheet width direction is projected onto the sheet material surface by using a slide on which the high-density linear pattern is drawn, this linear pattern is photographed by a camera, and based on the distortion of the linear pattern in the picked-up image thus obtained, the surface shape, and therefore the flatness of the sheet material is measured. In this method, unlike the method described in JP61-40503A, projection of the high-density linear pattern increases the measurement resolution (space resolution) of the surface shape, so that it can be expected that the surface shape of the sheet material can be measured with high accuracy.
The shape measuring method as described in JP2008-58036A is generally called a grating pattern projection method, and is used widely in various applications, being not limited to the case where the surface shape of a steel sheet is measured.
FIG. 1 is a schematic view showing a configuration example of a device for carrying out the grating pattern projection method. As shown in FIG. 1, in the grating pattern projection method, a grating pattern is projected onto the sheet material surface from the slantwise upper side with respect to the sheet material surface by a projector provided with a light source, a slide on which the grating pattern (generally, a linear pattern) is drawn, and an imaging lens. Then, the grating pattern projected onto the sheet material surface is photographed from the direction different from the grating pattern projecting direction by a two-dimensional camera. At this time, if the surface shape of the sheet material changes, an inclination angle of the sheet material surface also changes, and the pitch of the grating pattern in the picked-up image photographed by the camera (generally, the space between the light lines composing the linear pattern) changes according to the inclination angle of the sheet material surface. The relationship between the inclination angle of the sheet material surface and the pitch of the grating pattern in the picked-up image can be determined geometrically. Therefore, if the pitch of the grating pattern in the picked-up image is measured, the inclination angle of the sheet material surface can be calculated based on this measurement result and the above-described relationship. If the calculated inclination angle is integrated, the surface shape of the sheet material can be calculated.